Welcome! Today, I am excited to guide you through an intriguing task involving arrays: pairing up 'opposite' elements. Apart from arrays, we will dive deeper into learning about lists in C#. Lists offer more flexibility than arrays because they can dynamically resize and provide various useful methods for manipulating the collection of elements. This lesson will also include deeper explorations with tasks, providing an excellent opportunity to refine your array-handling and list-handling skills. Are you ready to start? Let's dive right in!

Before diving into the main lesson, let's briefly understand what a list is in C#. A list is a collection of objects that can be dynamically resized. Unlike arrays, lists are part of the System.Collections.Generic namespace and provide more flexibility:

• Dynamic Resizing: Lists can grow or shrink in size as needed.
• Additional Methods: Lists offer a rich set of methods such as Add, Remove, Insert, and more for easier manipulation of elements.

Here’s an example of how to declare and use a list in C#:

C#
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1using System;
2using System.Collections.Generic;
3
4class Program
5{
6    static void Main()
7    {
8        List<int> numbers = new List<int> {1, 2, 3, 4, 5};
9        
10        // Add: Appends an element to the end of the list.
11        numbers.Add(6); // The list now is {1, 2, 3, 4, 5, 6}
12
13        // RemoveAt: Removes an element at a specified index. It accepts the index of the element to be removed.
14        numbers.RemoveAt(0); // Removes element at index 0. The list now is {2, 3, 4, 5, 6}
15
16        // Remove: Removes the first occurrence of a specific element.
17        numbers.Remove(3); // Removes the element '3'. The list now is {2, 4, 5, 6}
18
19        // Insert: Inserts an element at a specified index. Accepts the index and the element to be inserted.
20        numbers.Insert(1, 9); // Inserts '9' at index 1. The list now is {2, 9, 4, 5, 6}
21        
22        foreach (int number in numbers)
23        {
24            Console.WriteLine(number);
25        }
26    }
27}

Our task is to form pairs of 'opposite' elements in a given array of integers. In an array of n elements, we view the first and last elements as 'opposite,' the second and second last elements as 'opposite,' and so on. If the array length is odd, the middle element is its own 'opposite.'

You are provided with an array of n integers, with n ranging from 1 to 100, inclusive. The task necessitates that you return a list of arrays, where each array comprises a pair of an element and its 'opposite' element.

For example, for int[] numbers = {1, 2, 3, 4, 5, 6, 7}, the output should be Solution(numbers) = {{1, 7}, {2, 6}, {3, 5}, {4, 4}, {5, 3}, {6, 2}, {7, 1}}.

Before we start writing code, let's familiarize ourselves with how to access the elements of an array in C#.

In C#, the i-th element of an array numbers can be accessed as numbers[i], with the index starting from 0. Consequently, the first element can be accessed using numbers[0], the second using numbers[1], and so forth, up to numbers[numbers.Length - 1] for the last element.

Already dressed to the nines? Brilliant! Now, let's figure out how to access each element's 'opposite.'

The 'opposite' of the i-th element of the array is the numbers.Length - i - 1-th element. To visualize this, imagine that you are standing at the start of a line and your friend is at the end of the line, with both of you being 'opposites.' So, the opposite element for numbers[0] is numbers[numbers.Length - 0 - 1], the opposite to numbers[1] is numbers[numbers.Length - 1 - 1], and so on.

Now, let's start coding our solution. We initiate by initializing an empty list in which we will store our 'opposite' pairs and by calculating the array's length for later use.

C#
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1using System;
2using System.Collections.Generic;
3
4public class Program
5{
6    public static List<int[]> Solution(int[] numbers)
7    {
8        int n = numbers.Length;
9        List<int[]> result = new List<int[]>();
10    }
11}

Next, we loop over all elements in our array.

C#
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1using System;
2using System.Collections.Generic;
3
4public class Program
5{
6    public static List<int[]> Solution(int[] numbers)
7    {
8        int n = numbers.Length;
9        List<int[]> result = new List<int[]>();
10        for (int i = 0; i < n; i++)
11        {
12            // next step comes here
13        }
14        return result;
15    }
16}

Within our loop, we create an array for each pair of 'opposite' elements. This pair includes the i-th element and the n - i - 1-th element. We then add this array to our result list. Here's the final version of our function:

C#
Copy to clipboard
1using System;
2using System.Collections.Generic;
3
4public class Program
5{
6    public static List<int[]> Solution(int[] numbers)
7    {
8        int n = numbers.Length;
9        List<int[]> result = new List<int[]>();
10        for (int i = 0; i < n; i++)
11        {
12            result.Add(new int[] { numbers[i], numbers[n - i - 1] });
13        }
14        return result;
15    }
16
17    public static void Main(string[] args)
18    {
19        int[] numbers = {1, 2, 3, 4, 5, 6, 7};
20        List<int[]> pairs = Solution(numbers);
21        
22        foreach (var pair in pairs)
23        {
24            Console.WriteLine($"({pair[0]}, {pair[1]})");
25        }
26    }
27}

This function iterates over all elements of the array and, for each element, forms a pair with its 'opposite' and stores the pair as an array in the result list.

You've done incredibly well! In this lesson, we explored what lists are in C#, understanding their dynamic nature and flexibility compared to arrays. Furthermore, we mastered the technique of pairing opposite elements in lists and honed our skills in list indexing. Keep practicing these concepts to solidify your understanding and enhance your coding proficiency. Happy coding!